Carbohydrates comprise about 50% of the mass of gp120, the external envelope glycoprotein of simian immunodeficiency virus (SIV) and human immunodeficiency virus We identified 11 replication-competent derivatives of SIVmac239 lacking two, three, four, or five potential sites for N-linked glycosylation These sites were located within and around variable regions 1 and 2 of the surface envelope protein of the virus Asn (AAT) of the canonical N-linked glycosylation recognition sequence (Asn X Ser/Thr) was changed in each case to the structurally similar Gln (CAG or CAA) such that two nucleotide changes in the codon would be required for reversion Replication of one triple mutant (g456), however, was severely impaired A revertant of the g456 mutant was recovered form CEMx174 cells with a Met-to-Val compensatory substitution at position 144, 2 amino acids upstream of attachment site 5 Thus, a debilitating loss of sites for N-linked glycosylation can be compens ated for by amino acid changes not involving the Asn-X-Ser/Thr consensus motif These results provide a framework to begin testing the hypothesis that carbohydrates form a barrier that can limit the humoral immune responses to the virus PUBLICATIONS Reitter, JN and Desrosiers, RC Identification of a replication-competent strains of simian immunodeficiency virus lacking multiple attachment sites for n-linked carbohydrates in variable regions 1 and 2 of the surface envelope protein J Virol 1998; 72:5399-5407